Water Resources Engineering - Research Paper Example

Abstract This report gives the results of the process of formation of bed forms in a laboratory set up. The conditions in the channels were changed through increasing the water pressure and the slope of channel and the changes were recorded the flow being sub-critical. The slope was changed from 0% to 25 and then to a maximum of 50%. At the 0% slope there was no recorded movement of bed materials. At stage two of water flow the bed form change with ripples being established and with increased slope the ripples were transformed to dunes the flow being at critical stage. The water was then adjusted flow was adjusted to the third stage and with the flow at this point being supercritical. Table of contents Abstract Introduction Apparatus Methods Results Discussion Conclusion References Introduction Ripples and dunes are fascinating natural sedimentary features which usually appear when sand is transported by air or water. They are therefore associated with deserts, oceans, beaches and rivers. Even though this phenomenon has been seen for along time it is only recently that the processes and mechanisms of fluid flow and sand transport that result into formation of these features has been understood ( Vanoni, 1975). The study of bedforms using flowing water instead of air was first done at Colorado State University in 1960s. This involved building a very large flume at Foothills Engineering Research Centre through the funding United States Geological Service. The experiments which were done by Daryl B. Simons and Everett V. Richardson in the large flumes are the basis of the current understanding of bedforms in rivers and canals (Raudkivi, 1967). Apparatus The equipments that were required for this experiment were Armfield S8 MkII Clean sand of a uniform size with size range of 0.1 to 0.3mm Hook and point gauge Methods The sand of a uniform size between 0.1mm and 0.3mm was placed in the flume to a uniform depth level with the top of the overspill weir. The sand was tamped down to ensure that there was uniform parking, and then there was slight compaction of the sand to ensure it was as flat as possible. The collecting/settling tank was then filled with clean water up to the full mark. Theory When water is flowing in channels two forces that oppose each other are in play. When the flow of fluid is uniform the two forces will be balanced such that their will be neither acceleration nor deceleration in accordance to Newton’s laws of motion. The downward force is due to the weight of the fluid while the resisting force is the friction force developed between water and the sediment forming the bed and the banks although the banks contribution maybe negligible. This experiment explores what happens in the channel as the water pressure and slope are increased Results The first step involved the slope of the machine being set at level 1 at which point the slope was at zero. The pump was then put on so that water could flow on the sand bed. The gradient was increases gradually with caution being taken to ensure that the threshold of bed material motion was not exceeded. The observation made at this point was that an increase in the speed of water did not result into the bed material being mobile. There were no any form of bed forms with the configuration of the bed being what is describe as plane bed with no motion. The bed slope was further increased gradually to a point where there was a small but finite sediment load. The discharge was increased to level 2. This increased the depth of flow of water as well as the rate of sediment movement. Immediately after the increase there were formation ripples which are small triangular bed forms. The ripples were a few millimeters high and their widths ranged from about 1 cm to 2cm. The ripples were asymmetrical with their upstream slope being gentle their downstream appeared steeper. The formation of the ripples involved the movement of sediments that involved rolling and sliding motion with the contact with the bed being constant. There slope was also increased gradually. The grains were seen to hop and bounce but the predominant mode of movement of the grains was still rolling. As the intensity of flow increased by increase of slope there was formation of dunes which are larger bedforms compared ripples. The dunes were about 100mm long and had a height range of between 10-15mm. Although the shape of the dunes were as triangular as those of the ripples, they were slightly longer and the top was flatter. A closer observation revealed that water surface was drawn down to some extent on the crest of the dune but was humped at the troughs between the dunes a clear indication that the waves on water surface was out of phase with bed waves. This is a clear indication the water flow is sub-critical and is said to belong to lower regime The discharge was increased to level 3. This will result into an increased flow depth with accompanied increase in intensity of sediment motion. Saltation was seen to become more pronounced with some grains being observed to leap from crest of one dune directly to stoss slope of the next dune with no rest in the trough. At this point it was observed that there was growth of the dunes height while the length remained approximately constant in addition to their rate of downward progression being increased. When the discharge it at stage three it is at its maximum setting and in order to increase flow and sediment transport intensity the slope is increased. When the slope is increased the large dunes were seen to increase in length while their amplitude reduced. The dumes with longer lengths and reduced heights are referred to wash out dunes. The dunes at this stage retained their triangular shape but did not have deep troughs and scour holes seen in large dunes. The bedload motion adapted the appearance of a carpet of grains in motion with streaks being seen to appear in the pattern of bedload. The boils as well as drawdown areas in the water surface disappeared and inside the bed some of the cross bedding that was formed in the dune regime were destroyed with replacement with long and low angled layers of sand. The washed out dune then quickly occupied the whole working length. A further increase in slope transformed the flat and smooth water surface into undulations referred to as standing waves which were practically stationary as suggested by the name. The bed also developed waves which were symmetrical and stationary unlike those formed in the ripples and dunes. The waves on the water surface and on the bed were observed to be in phase which was an indication of critical or super-critical flow also referred to as upper regime. An increase in slope resulted into a substantial increase in the amplitude of the surface waves and also an increased size of the bedforms. At this point the bed and the water surfaces were in phase with the flow at this point being supercritical. Discussion In this experiment there were so many changes which were observed in the channel as the slope and water depth were observed. When the discharge was increased to level two, there was hoping and bouncing of the grains. This was as a result of grains movement being as suspended load, being held up in the flowing water body through turbulent velocity fluctuations, which is unlike with the case of the saltation grains where their weight is not borne from solid to solid contact at the bed. The hopping type of movement will be experienced when grains are lifted off the bed through fluid turbulence with motion being by saltation (Vanoni, 1975). . The path of saltation grains conform to ballistic like trajectories a clear indication that they are not supported by fluid forces in the saltation process and that the particles submerged weight actually receive support by bouncing contact with the bed. When the water flow was increased at water three it was observed that ripples were transformed to dunes. During this stage grains movement is as suspended load, being held up in the flowing water body through turbulent velocity fluctuations, which is unlike with the case of the saltation grains where their weight is not borne from solid to solid contact at the bed (Raudkivi, 1967). . At the time of establishment of a uniform dune bed along the flume, a substantial amount of sediment in the bed had been reworked by the passing of dunes which are named cross bedding. It is important to note at this point that some of the changes may not be noticeable depending on the keenness of those performing the experiment and the scale of apparatus in use. Conclusion This experiment has demonstrated the process formation and it has clearly been demonstrated that the features which are formed in nature can created in a laboratory set up with their causes clearly being established. References Raudkivi A.J. (1967). Loose boundary hydraulics. Pergamon Press Leliavsky S. (1955). An introduction to fluvial hydraulics. Constable Linsley R.K. & Franzini J. B. Water resources engineering. MacGraw-Hill Inter Henderson. F. M. (1966). Open channel flow. MacMillan Company, New York. Vanoni V. (1975). Sedimentation Engineering. Asce Technical manual 54. Read More